1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Related Art
An image forming apparatus employing xerography has been known. Such an image forming apparatus of xerography forms a toner image on a recording material (paper) in the following steps. First, a charging device charges an image supporting body (for example, a photoreceptor drum) in advance. Next, an exposure device irradiates a surface of the photoreceptor drum with lazer light. In this way, an electrostatic latent image is formed on a surface of the photoreceptor drum. In addition, by applying a developing bias voltage in a developing device supporting toner, the toner that is charged in advance attaches to the electrostatic latent image to form a toner image.
Subsequently, a transfer unit transfers the toner image to the recording material directly or via an intermediate transfer belt. A fixing unit fixes the toner image transferred to the recording material onto the recording material.
The image forming apparatus thus forms an image on a surface of the recording material.
In order to form an image stably and without unevenness on the surface of the recording material, it is necessary to control toner density of the toner image formed on the image supporting body at a predetermined stage.
Given this, a technology of calibrating image density using a density sensor is disclosed (related Art 1). More specifically, in an image forming apparatus that forms an image on a recording material via an intermediate transfer belt, the image forming apparatus forms a reference image (for example, a toner patch) on a photoreceptor drum and transfers the reference image to the transfer belt. Next, the image forming apparatus detects a toner amount of the reference image by irradiating the reference image with light. And then, the image forming apparatus obtains a calibration amount for toner density, based on a result of detection.
However, in a case of an image forming apparatus that transfers a toner image directly to the recording material, upon transfer of the toner image to the recording material, toner migrating from the photoreceptor drum to the recording material may attach to a conveying surface of a conveying belt that conveys the recording material (for example, fogging toner which is attaced on photoreceptor drum in a interval of conveyning sheets of paper). Such a residual toner attached to the conveying surface may contaminate a reverse face of the recording material.